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A few months back I drew up a TTS/R8 style spindle to make sure the machine I am building would allow me to upgrade should I decide to go that way. Here is an overview of the basic parts:

I'm probably 6 months away from building this, if I ever do, but Chaz asked for more info about it recently so here are a few sketches and the key parts as IGES files (see attachment). Fusion 360 can read these no problem so you can have a look.

I drew this quite quickly and have not spent time on the details - for example there is no labyrinth seal at the bottom of the spindle to stop the swarf getting into the bearings. Also the upper bearings are a simple sliding fit in the housing to allow for expansion but some designs have a wavy washer to allow movement but keep a small pre-load to prevent the bearing chattering.
For info I was planning to run it off my WC spindle via pulley(s).

Anyway here it is, maybe it will give you a place to start or some ideas.

Some of the advantages of this system are:
1. Pulley driven so allows step down of speed, step up of torque
2. Lower speed and higher torque allows larger diameter tools to hog material away
3. Option to add a power draw bar for quick tool changes (air cylinder arrangement squeezes the draw bar against the spindle shaft to push the collet out)
4. Uses an off-the-shelf R8 collet (3/4")
5. Uses an off-the-shelf TTS holder
6. Main body is 80 mm diameter so could drop into standard WC spindle mount
7. Tools remain in TTS holder which references against a shoulder when fitted so Z height is known